This invention relates to power amplifiers and, in particular, to power amplifier that are capable of delivering large amounts of current to inductive loads at relatively high repetition rates.
A power amplifier that is designed to drive an inductive load typically operates as a high-current switching device. One such power amplifier of this type is known as a resonant scan deflection circuit and is used to drive the horizontal deflection yoke of a cathode-ray tube.
A conventional resonant scan deflection circuit is shown in FIG. 1. The operation of this circuit may be described in the following three cycles:
A. The transistor Q is turned on. The transistor receives collector current from the deflection yoke L1 (due to charge previously stored in the S-capacitor C1) and from the decoupling inductor L2. The collector current ramps up from zero towards some maximum value. The voltage at the node A is close to ground.
B. The transistor Q is turned off and a resonant circuit consisting primarily of the deflection yoke L1 and the flyback capacitor C2 is formed. The voltage at the node A follows the first half of a cycle of a sine wave. At the beginning of the second half of the cycle of the sine wave, when the voltage at the node A goes negative, the damper diode D1 becomes conductive and damps the operation of the resonant circuit.
C. The voltage at the node A is negative. Current flows through the damper diode D1 to the deflection yoke L1 and the decoupling inductor L2. The current in the yoke L1 ramps up from a negative value to zero. Base current is supplied to the transistor Q well before the end of cycle C in order to ensure that the transistor is conductive before the end of cycle C.
In accordance with the disclosure of copending application Ser. No. 651,732 filed Sept. 17, 1984, an isolation diode may be connected between the collector of the transistor Q and the node A in order to prevent the collectorbase junction of the transistor from turning on during cycle C due to the negative voltage at the node A, and thereby robbing the transistor of the base current at the end of cycle C.